Reverse engineer little planet to equirectangular image?

[SunDance]

What I love most about this forum (and others) is how we learn from one another and how generous so many experienced photographers are about sharing their techniques, secrets, etc.

 

Often I look at a VR photo, or a little planet, and wonder precisely how it was made. Specifically, I wonder where the photographer was standing, or where his tripod was, before they were “erased” from the published image.

 

All this is by way of background to asking a question. I know how to use image manipulation software to “defish” an image shot with a fisheye or other very wide-angle lens. Is there a way to use PTGui to take a “little planet” image and reverse engineer it, working backward so I can see the 180 x 360 equirectangular image from which it was made?

[Erik Krause]

Am 22.11.2012 21:36, schrieb SunDance:
> All this is by way of background to asking a question. I know how to use

> image manipulation software to �defish� an image shot with a fisheye or

> other very wide-angle lens. Is there a way to use PTGui to take a

> �little planet� image and reverse engineer it, working backward so I can

> see the 180 x 360 equirectangular image from which it was made?

A "Little Planet" never covers the full 360�x180� of an equirectangular
image. So you would get a spherical image with a zenith hole. However,
PTGui has no stereographic input projection. You'd need to use fisheye
input and hope the distortion parameters would do.

I made a little planet of one of my panoramas, then loaded the result as
a second image with individual lens parameters and tried to align it to
the original panorama. This works more or less, well actually rather less...

Unfortunately PTGui doesn't find control points automatically in
equirectangular source images, so I had to place a couple of them
manually. Interestingly area control points where found after the planet
image roughly aligned, but only in the left half of the equirect.
Optimization leads to a bad result (don't re-initialze!).

So with the lens correction values of a=2.9, b=-4, c=2, pitch=-90 and
FoV of 300� you get a distorted approximation of the original equirect
if the little planet is the PTGui standard one. Could be you need to
vary FoV for others.

BTW.: hugin features stereographic input and converts the planet image
to a perfect equirectangular image (with zenith hole of course)...

--
Erik Krause
http://www.erik-krause.de

[John Houghton]

On Nov 22, 9:33 pm, Erik Krause <erik.kra...@gmx.de> wrote:
> A "Little Planet" never covers the full 360 x180 of an equirectangular
> image.

The Little Planet format that Pano2VR generates does cover 360x180.
It can be remapped back to equirectangular simply by loading it into
PTGui as a circular fisheye, fov 360, with the appropriate circular
crop that PTGui sets by default.

John

[Erik Krause]

Am 22.11.2012 22:46, schrieb John Houghton:
> The Little Planet format that Pano2VR generates does cover 360x180.
> It can be remapped back to equirectangular simply by loading it into
> PTGui as a circular fisheye, fov 360, with the appropriate circular
> crop that PTGui sets by default.

So those are round (circular cropped) images in fisheye projection? I
didn't see much of those lately. Actually they have 360�x360�.
Most I saw are square images with heavily stretched outer regions -
stereographic, like PTGui does. Stereographic can't cover 360�, the
image would be infinitely wide...

Both versions of the same source image can be seen on
http://wiki.panotools.org/Unusual_remappings

[Peter A. Schaible]

Wow! You guys a wicked, WICKED smart!

Erik Krause wrote:
> Am 22.11.2012 22:46, schrieb John Houghton:
>> The Little Planet format that Pano2VR generates does cover 360x180.
>> It can be remapped back to equirectangular simply by loading it into
>> PTGui as a circular fisheye, fov 360, with the appropriate circular
>> crop that PTGui sets by default.
>
> So those are round (circular cropped) images in fisheye projection? I

> didn't see much of those lately. Actually they have 360�x360�.

> Most I saw are square images with heavily stretched outer regions -

> stereographic, like PTGui does. Stereographic can't cover 360�, the

> image would be infinitely wide...
>
> Both versions of the same source image can be seen on
> http://wiki.panotools.org/Unusual_remappings
>

--

-- Peter

Peter A. Schaible

SunDance New Media
215 Stony Brook Road
Brewster, MA 02631

508/385-0055

[Tom Sharpless]

With Panini you can view a stereographic image as a spherical pano (though not save it as equirectangular). Same for cylindrical, Mercator, spherical, equirectangular and any kind of fish-eye photo.
-- Tom

On Thursday, November 22, 2012 5:23:37 PM UTC-5, SunDance wrote:

Wow! You guys a wicked, WICKED smart!

Erik Krause wrote:
> Am 22.11.2012 22:46, schrieb John Houghton:
>> The Little Planet format that Pano2VR generates does cover 360x180.
>> It can be remapped back to equirectangular simply by loading it into
>> PTGui as a circular fisheye, fov 360, with the appropriate circular
>> crop that PTGui sets by default.
>
> So those are round (circular cropped) images in fisheye projection? I

> didn't see much of those lately. Actually they have 360�x360�.

> Most I saw are square images with heavily stretched outer regions -

> stereographic, like PTGui does. Stereographic can't cover 360�, the

[zakato]

Revisiting this, now 10 year old, post to find answers on a LP to EQUI conversion (thank you all!).

So Hugin has had this Stereographic input option for years, but since PTGui 11 we can now also do this directly by simply setting the Fisheye Factor to Stereographic: k = 0.5.
 
(PTGui does not stop surprising me every single day! :)

Cheers everyone,

Antonio @zakato360

[Erik Krause]

Am 01.12.22 um 15:46 schrieb zakato:

> since PTGui 11 we can now also do this directly by simply setting the

> Fisheye Factor to Stereographic:*k* = 0.5.

Yes, if the little planet was done using stereographic projection, k =
0.5 would be right. However, you can also use any other fisheye
projection to make little planets. If it was done with an older version
of PTGui it is likely to be in equidistant projection (k=0). In other
words: you have to try...

--
Erik Krause